Many existing and proposed future wireless communication systems typically employ ARQ (Automatic Repeat Request) to repeat the transmission of data packets that were received in error. If a data packet is received correctly, a receiver transmits back an ACKnowledgement message (ACK) to a transmitter which proceeds by transmitting a new data packet. However if a data packet is received incorrectly, the receiver transmits a Negative ACKnowledgement (NACK) message. In response to receiving a NACK, the transmitter retransmits the same data packet a number of times until it receives an ACK. The maximum number of retransmissions of a data packet can be defined by the wireless communications system. A drawback of all ARQ systems is that they insert latency as there may be a considerable delay incurred due to the receiver having to signal “NACKs” and then wait for and decode multiple retransmissions of the same packet. Such delays can be particularly serious if retransmission is handled in higher layers.
Hybrid Automatic Repeat Request (HARQ) techniques have been applied to wireless communication systems to improve their performance and efficiency. In a HARQ system, a FEC (Forward Error Correction) code word is sent with each transmission and is used to determine if the packet has been received correctly. If a data packet is received correctly, an ACK is sent by the receiver thereby enabling the transmitter to transmit the next packet. However a NACK is issued in the event of the packet being received incorrectly. In response to the NACK, the transmitter resends the packet with a different part of the FEC code word and the receiver collects the received coded bits from all transmissions of the same packet before attempting decoding.
In order to increase the capacity of wireless communication systems, many wireless communication systems now use multiple antenna systems to transmit and/or receive information. The capacity of a system is the total amount of information conveyed by the communications system over a defined period of time. MIMO antenna systems are now proposed because they enable different information to be transmitted and to be received simultaneously. Integrating typical MIMO techniques with HARQ in an optimal way is not straightforward. This is illustrated in the following two examples.
In a first example Spatial multiplexing schemes, for example V-BLAST (for reference see Chung S. T., Lozano A., Huang H. C., “Approaching Eigenmode BLAST Channel Capacity Using V-BLAST with Rate and Power Feedback”, in Proc. IEEE Vehicular Technology Conf. (VTC Fall 2001) vol. 2, pp. 915-919, Atlantic City, N.J., October 2001, and Wolniansky P. W., Foschini G. J., Golden G. D., Lavenzuela R. A., “V-BLAST: An Architecture for Realising Very High Data Rates Over the Rich-Scattering Wireless Channel”, Proc. ISSSE 98, Pisa, September 1998); D-BLAST (for reference see Forschini G. J., “Layered Space-Time Architecture for Wireless Communication in a Fading Environment when using Multi-Antennas” Bell Labs Tech. J., pp 41-59, Autumn 1996); and PARC (for reference see Lucent, TSG-R1-01-0879, “Increasing MIMO Throughput with Per-Antenna Rate Control,” 3GPP TSG RAN WG1 Document, available through ftp://ftp.3gpp.org/, 2002; and Ericsson, “Selective Per Antenna rate Control (S-PARC), 3 GPP TSG RAN WG1, R1-04-0307), or closed-loop schemes (for reference see IST-2003-507581 WINNER, “Assessment of Advanced Beamforming and MIMO Technologies” D2.7, February 2005), use multiple transmit antennas to transmit a number of streams of data (“spatial sub-streams”) simultaneously via the same frequency channel. Typically these sub-streams are coded independently of each other, which allows rate adaptation of the sub-streams to the current channel conditions (as in for example PARC). Typically, if an error is detected in any one sub-stream then a retransmission will be requested for all sub streams. This is because the receiver possesses a combination of the sub-streams that it must attempt to separate, and errors in decoding one sub-stream will tend to coincide with errors in the other sub-streams.
In a second example some MIMO schemes split a code word across multiple antennas for transmission. Such schemes include “Spatial Channel Coding” (for reference see Philips, TSG-RAN1-04-0920, “Spatial Channel Coding for High Throughput With a Single Receive Antenna”, 3GPP TSG RAN WG1 Document, available through ftp://ftp.3gpp.org/, 2004), and space-time trellis coding (for reference see Tarokh V., Seshadri N., Calderbank A. R., “Space-Time Codes for High Data rate Wireless Communication: Performance Criterion and Code Construction2, IEEE Transactions on Information Theory, Vol. 44, No. 2, March 1998). Transmit diversity schemes (for example space-time block codes) can also be placed in this class by considering the diversity transmission to be a simple form of coding across antennas (the simplest approach being repetition coding). In such schemes ARQ can be straightforwardly applied by repeating unsuccessfully decoded code words. However application of HARQ-type II basically requires a further layer of coding, such that the “data” bits input to the spatial code across antennas are in fact the bits remaining after puncturing a low-rate HARQ code word. It should also be noted that, although potentially offering good performance, schemes that code across antennas are typically inflexible (the code is designed for a specific transmission rate and number of antennas) and may require high complexity decoding at the mobile terminal.
US Published Patent Specification 2004/0213184 A1 discloses that increased complexity can be avoided when a HARQ process is created for all the antennas. One coding process, that is, a single FEC coder is used across all the antennas and hence only a single block code will be generated for the antennas. Original information to be transmitted is coded by the channel coder, which operates at a fixed code rate and becomes coded information, referred to as a coded block. The coded block is then distributed as packets by a distribution unit among the plurality of antennas based on channel information received by a distribution unit and then rate matched and modulated before transmission.
The groups of coded sub-blocks are thus transmitted through one or more of the antennas. During subsequent retransmissions, the same coded block from the first transmission will be used and the number of sub-blocks in each group for each antenna will be re-calculated based on the channel condition of the antenna during the time of retransmission. Subsequently, the sub-blocks of each group are again rate matched and modulated to meet the current channel condition of the antenna through which the sub-blocks group are to be transmitted. The distribution unit retransmits previously transmitted information when a NACK is received but with newly selected number of sub-blocks in each antenna based on the current channel conditions of the antenna.
EP 1 298 829-A1 discloses HARQ techniques for multiple antenna systems in which at a transmitting station multiple error coded streams are formed from one block of information and transmitted by two or more antennas. Each error coded stream may be formatted according to the same protocol or different protocols, such as the Chase protocol and Incremental Redundancy (IR) protocol. If the information is not received correctly by a receiving station, a NACK is transmitted to the transmitting station. In response to receiving the NACK the multiple error coded streams are resent and at the receiving station the Chase protocol or IR protocol streams are combined with the previously sent streams. If a NACK is transmitted the process is repeated until either an ACK is transmitted or the process is timed-out and no more retransmissions of those multiple error coded streams are made. In the case of error coded streams formatted according to the IR protocol additional redundancy parity bits may be included in successive retransmissions.
US Published Patent Specification 2003/0072285 A1 discloses a HARQ technique using basis hopping for MIMO systems. The technique changes the basis (V) upon retransmission, which helps reduce the error probability upon retransmission. The idea behind changing the basis upon retransmission is the fact that the error rate performance of the MIMO scheme is affected by the choice of basis (V). When a packet is declared in error, choosing a different basis will likely reduce the error probability upon retransmission.
In order to minimise retransmission delays and thereby reduce latency, it is desirable that the probability of the first transmission failing is very low. This can be difficult to achieve in unfavourable propagation conditions or in radio links with limited ability to adapt their transmissions to the current channel characteristics.